1. Field of the Invention
This invention relates to a rear wheel drive vehicle of the type having a pair of front wheels which are steering wheels, a pair of rear wheels which are steerable driving wheels, and a differential gear means coupled between the respective axles of the two rear wheels, the differential gear means including a differential restricting means.
2. Description of the Prior Art
It has been hitherto known, in the above type of vehicle, that the differential gear means is a differential lock type having a differential restricting means, so that when the rear wheel on one side is in a low friction condition such as a muddy condition or the like, the lowering of the driving force of the rear wheel on the other side may be restricted.
In a rear wheel drive vehicle having a differential gear means of a differential lock type including a differential restricting means, if when the vehicle is moving, as shown in FIG. 1a, an external force Fx caused by a side wind, a roughness of the surface of a road or the like is applied to the vehicle, the vehicle begins to turn clockwise as a result of the moment Mx about the center of gravity M of the vehicle. The rear wheel situated on the inside of the turning direction has a differential restricting torque F1 applied thereto, due to a difference in turning speed between the inside rear wheel and the rear wheel situated on the outside thereof, that is a result of the differential restricting means. Thus, the moment M1 of the differential restricting torque F1 is multiplied by a distance l.sub.R measured from the center of gravity M to the line of the driving force of the rear wheel on the inside, and is applied counterclockwise about the center of gravity M of the vehicle. Accordingly, this moment M1 acts to cancel the moment Mx which is caused by the external force Fx which turns the vehicle clockwise, thereby obtaining the advantage of an improvement in the straight running stability of the vehicle against external disturbances. In this case, the larger the differential restricting torque caused by the differential restricting means, the larger the straight running stability.
When steering of the vehicle, however, as shown in FIG. 1b, the rear wheel on the inside of the turning direction has a restricting torque applied thereto which is almost the same differential restricting torque F1 as above, and a moment M1 about the center of gravity M of the vehicle, caused by this differential restricting torque F1, acts on the vehicle in a direction which is opposite to the steering or turning direction of the vehicle. The torque is caused by the cornering force of the front wheels, and results in a comparatively strong under steering.
Additionally, when steering the vehicle, if the rear wheel on the inside of the steering direction slips, as shown in FIG. 1c, the rear wheel on the outside of the steering or turning direction has the differential restricting torque F1 applied thereto, due to a difference between the turning speed thereof and that of the rear wheel on the inside thereof. This is a result of the action of the differential restricting means. A moment M1 about the center of gravity M of the vehicle caused by the differential restricting torque F1, acts on the vehicle in the same direction as the steering or turning direction of the vehicle caused by the cornering force on the front wheels, so that there is a resulting comparatively strong over steering.
This tendency is increased with an increase in the differential restricting torque of the differential restricting means. Consequently, it becomes difficult to obtain both straight running stability and steering stability.